Losing the battle against fungal infection: Suppression of termite immune defenses during mycosis

The progression of M. anisopliae fungal infection on the cellular immune defenses of the dampwood termite Zootermopsis angusticollis was studied by quantifying the number and types of circulating hemocytes of naïve nymphs and nymphs exposed to either a control suspension lacking conidia, 2x10³, 2x10⁶ or 2x10⁸ conida/ml doses. Hemocyte density was monitored on days 1, 2, 3, 4, 7 post-exposure. Our results show that the density of prohemocytes and particularly plasmatocytes, but not granular hemocytes, changed as a function of both conidia dosage and time elapsed since exposure. The development of mycosis beyond the third day resulted in an almost complete collapse of plasmatocytes which coincided with the appearance of hyphal bodies in the hemolymph and the onset of “sluggish” behavior, culminating in termite death. Prophenoloxidase activity (PO), used as a proxy for estimating investment in immunity function, surged three days after exposure to a 10⁸ conidia/ml and then plummeted by day seven. Thus, the initial immune response in these termites is overtaken by M. anisopliae, mainly by destroying the host’s plasmatocytes and reducing PO activity that is necessary for the successful encapsulation of the invading fungus.